The field of art to which this invention pertains is the hydrocracking of a hydrocarbonaceous feedstock. Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline by hydrocracking a hydrocarbon feedstock derived from crude oil, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 700xc2x0 F., usually at least about 50 percent by weight boiling above 700xc2x0 F. A typical vacuum gas oil normally has a boiling point range between about 600xc2x0 F. and about 1050xc2x0 F.
Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts within a hydrocracking reactor influence the yield of the hydrocracked products.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs, higher liquid product yields and improved operability.
U.S. Pat. No. 5,720,872 (Gupta) discloses a process for hydroprocessing liquid feedstocks in two or more hydroprocessing stages which are in separate reaction vessels and wherein each reaction stage contains a bed of hydroprocessing catalyst. The liquid product from the first reaction stage is sent to a low pressure stripping stage and stripped of hydrogen sulfide, ammonia and other dissolved gases. The stripped product stream is then sent to the next downstream reaction stage, the product from which is also stripped of dissolved gases and sent to the next downstream reaction stage until the last reaction stage, the liquid product of which is stripped of dissolved gases and collected or passed on for further processing. The flow of treat gas is in a direction opposite the direction in which the reaction stages are staged for the flow of liquid. Each stripping stage is a separate stage, but all stages are contained in the same stripper vessel.
International Publication No. WO 97/38066 (PCT/US 97/04270) discloses a process for reverse staging in hydroprocessing reactor systems.
U.S. Pat. No. 3,328,290 (Hengstebech) discloses a two-stage process for the hydrocracking of hydrocarbons in which the feed is pretreated in the first stage.
U.S. Pat. No. 5,114,562 (Haun et al) discloses a process wherein a middle distillate petroleum stream is hydrotreated to produce a low sulfur and low aromatic product employing two reaction zones in series. The effluent of the first reaction zone is cooled and purged of hydrogen sulfide by stripping and then reheated by indirect heat exchange. The second reaction zone employs a sulfur-sensitive noble metal hydrogenation catalyst. Operating pressure and space velocity increase, and operating temperature decreases from the first to the second reaction zones. The ""562 patent teaches that the hydroprocessing reactions of the hydrodenitrification and hydrodesulfurization will occur with very limited hydrocracking of the feedstock. Also, it is totally undesired to perform any significant cracking within the second reaction zone.
U.S. Pat. No. 5,120,427 (Stine et al) discloses a hydrocracking process wherein the product fractionation zone produces a net bottoms stream comprising polynuclear aromatic compounds. The liquid recycle stream from the fractionation zone is produced from a point above the feed point. The ""427 patent fails to disclose a divided-wall fractionator to produce both a liquid recycle stream and a small drag stream from the bottom of the fractionator.
The present invention is a catalytic hydrocracking process which uses a divided-wall fractionator to recover lower boiling hydrocarbon product streams, a liquid recycle stream and a drag stream containing a high concentration of heavy polynuclear aromatic compounds. The process of the present invention benefits from the ability to achieve a lower capital cost, lower operating expense and simplified operation.
In accordance with one embodiment, the present invention relates to a process for hydrocracking a hydrocarbonaceous feedstock which process comprises: (a) passing a hydrocarbonaceous feedstock, a liquid recycle stream and hydrogen to a hydrocracking zone containing hydrocracking catalyst; (b) partially condensing the effluent from the hydrocracking zone to produce a hydrogen-rich gaseous stream and a first liquid hydrocarbonaceous stream; (c) introducing at least a portion of the first liquid hydrocarbonaceous stream comprising hydrocarbons boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds into a first zone of a divided-wall fractionation zone to produce at least one liquid hydrocarbonaceous product stream and a second liquid hydrocarbonaceous stream comprising hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds; (d) reintroducing at least a portion of the second liquid hydrocarbonaceous stream into a second zone located in the bottom end of the divided-wall fractionation zone to produce a third hydrocarbonaceous stream rich in polynuclear aromatic compounds; (e) recycling at least another portion of the second liquid hydrocarbonaceous stream to the hydrocracking zone to provide at least a portion of the liquid recycle stream; and (f) recovering the liquid hydrocarbonaceous product stream.
In accordance with another embodiment, the present invention relates to a process for hydrocracking a hydrocarbonaceous feedstock which process comprises: (a) passing a hydrocarbonaceous feedstock, a liquid recycle stream and hydrogen to a denitrification and desulfurization reaction zone containing a catalyst and recovering a denitrification and desulfurization reaction zone effluent therefrom; (b) passing the denitrification and desulfurization reaction zone effluent to a hydrocracking zone containing hydrocracking catalyst; (c) partially condensing the reaction zone effluent from step (b) to produce a hydrogen-rich gaseous stream and a first liquid hydrocarbonaceous stream; (d) passing the first liquid hydrocarbonaceous stream to a flashing zone having a reduced pressure to produce a first gaseous stream comprising hydrogen and normally gaseous hydrocarbons and a second liquid hydrocarbonaceous stream; (e) stripping the second liquid hydrocarbonaceous stream to produce a second gaseous stream comprising normally gaseous hydrocarbons and a third liquid hydrocarbonaceous stream comprising hydrocarbons boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds; (f) fractionating the third liquid hydrocarbonaceous stream in a first zone of a divided-wall fractionation zone to produce at least one liquid hydrocarbonaceous product stream and a fourth liquid hydrocarbonaceous stream comprising hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds; (g) reintroducing at least a portion of the fourth liquid hydrocarbonaceous stream into a second zone located in the bottom end of the divided-wall fractionation zone to produce a fifth hydrocarbonaceous stream rich in polynuclear aromatic compounds; (h) recycling at least another portion of the fourth liquid hydrocarbonaceous stream to the denitrification and desulfurization reaction zone to provide at least a portion of the liquid recycle stream; and (i) recovering the liquid hydrocarbonaceous product stream.
Other embodiments of the present invention encompass further details such as types and descriptions of feedstocks, hydrocracking catalysts and preferred operating conditions including temperatures and pressures, all of which are hereinafter disclosed in the following discussion of each of these facets of the invention.